This application claims priority to Application No. DE 100 59 645.2 which was published in the German language on Dec. 1, 2000.
The present invention relates to a system and method for synchronizing base stations coupled to switching system parts in a radio network.
Present-day communications systems are increasingly communicating in a wireless environment. Terminals in the system may be used for different purposes, such as for transmitting speech, video, fax, data files, programs and/or measurement data. A number of different radio network technologies have been developed for this purpose in different fields of application, such as GSM (Global System for Mobile Communication) and DECT (Digital Enhanced Cordless Telephony). Generally, mobile terminals are wirelessly linked via one or more base stations, which are themselves connected to switching devices or switching system parts.
In order to supply a larger area with wireless connection capabilities, a number of base stations are generally distributed over the area. The base stations have radio cells which form a mobile radio network covering the area. A subscriber can move freely within the area of coverage, and an existing connection can be passed on without any interruption from one radio cell to the next, or from one base station to the next base station. A change in the connection routing without any interruption (i.e. No interruption perceived by the subscriber) between base stations is referred to as a seamless handover.
In DECT systems, carrying out such a seamless handover places very stringent requirements on the synchronicity of the base stations involved. For example, adjacent base stations in a DECT mobile radio network must maintain a frequency accuracy of at least +/xe2x88x925 ppm for a correct seamless handover procedure. Furthermore, they must keep their respective radio frames (10 ms) and radio superframes (600 ms and 800 ms) synchronized to one another. The mutual error between the radio frames must not exceed +/1.8 microseconds.
In distributed mobile radio networks which have a number of switching system parts, the required synchronicity can be ensured only with considerable additional complexity. Thus, for example, mobile radio networks that are currently being developed provide for each switching system part to be externally supplied with high-precision frequency and time information via a GPS receiver (Global Positioning System) in order to produce synchronicity between DECT base stations covering switching nodes. However, the required GPS receiver represents a considerable additional cost factor.
In one embodiment of the invention, there is a method for synchronization of base stations in a mobile radio network. The method comprises, for example, transmitting a first synchronization signal from a first switching system part, causing a first base station, coupled thereto, to transmit radio frames which are aligned in time with the first synchronization signal, and transmitting a second synchronization signal from a second switching system part, causing a second base station, coupled thereto, to align its radio frame clock with the second synchronization signal, wherein the second base station receives the transmitted radio frames and determines a time error with respect to the radio frame clock, and transmits the time error to the second switching system part, and the second switching system part transmits a third synchronization signal to the second base station, with a transmission time selected as a function of the transmitted time error such that the radio frame clock of the second base station is synchronized with the radio frames of the first base station by time alignment with the third synchronization signal.
In one aspect of the invention, when selecting the transmission time for the third synchronization signal, a delay time between the second switching system part and the second base station is taken into account as a compensating factor.
In another aspect of the invention, the first and the second synchronization signals are each transmitted during startup of the first and second switching system parts, respectively.
In still another aspect of the invention, during startup of the first and the second switching system parts, the first switching system part is configured as a synchronization source, and the second switching system part is configured as a synchronization sink.
In another aspect of the invention, the first base station is configured by the synchronization source as a radio frame transmitter, and the second base station is configured by the synchronization sink as a radio frame receiver.
In another aspect of the invention, the second switching system part transmits an identifier for the first base station to the second base station, and the second base station uses the transmitted identifier to selectively receive the radio frames from the first base station.
In yet another aspect of the invention, when the second base station has been synchronized, the second switching system part is configured as a synchronization source for additional switching system parts in the mobile radio network.
In another aspect of the invention, the second base station uses radio frames received during operation to determine a drift in the radio frame clock of the second base station, and transmits drift information to the second switching system part, and the second switching system part uses the transmitted drift information to derive correction information and transmits the information to at least one additional base station, which is coupled to the second switching system part and readjusts the radio frame clock of the base station on the basis of the transmitted correction information.
In another embodiment of the invention, there is a system for synchronization of base stations in a mobile radio network. The system includes, for example, a first switching system part, to transmit a first synchronization signal, causing a first base station, coupled to the first switching system part, to transmit radio frames which are aligned in time with the first synchronization signal, and a second switching system part to transmit a second synchronization signal, causing a second base station, coupled to the second switching system, to align its radio frame clock with the second synchronization signal, wherein the second base station receives the transmitted radio frames and determines a time error with respect to the radio frame clock, and transmits the time error to the second switching system part, and the second switching system part transmits a third synchronization signal to the second base station, with a transmission time selected as a function of the transmitted time error such that the radio frame clock of the second base station is synchronized with the radio frames of the first base station by time alignment with the third synchronization signal.
In one aspect of the invention, when selecting the transmission time for the third synchronization signal, a delay time between the second switching system part and the second base station is taken into account as a compensating factor.
In another aspect of the invention, the first and the second synchronization signals are each transmitted during startup of the first and second switching system parts, respectively.
In still another aspect of the invention, during startup of the first and the second switching system parts, the first switching system part is configured as a synchronization source, and the second switching system part is configured as a synchronization sink.
In another aspect of the invention, the first base station is configured by the synchronization source as a radio frame transmitter, and the second base station is configured by the synchronization sink as a radio frame receiver.
In another aspect of the invention, the second switching system part transmits an identifier for the first base station to the second base station, and the second base station uses the transmitted identifier to selectively receive the radio frames from the first base station.
In yet another aspect of the invention, when the second base station has been synchronized, the second switching system part is configured as a synchronization source for additional switching system parts in the mobile radio network.
In another aspect of the invention, the second base station uses radio frames received during operation to determine a drift in the radio frame clock of the second base station, and transmits drift information to the second switching system part, and the second switching system part uses the transmitted drift information to derive correction information and transmits the information to at least one additional base station, which is coupled to the second switching system part and readjusts the radio frame clock of the base station on the basis of the transmitted correction information.